Swollen starch-latex compositions for use in papermaking

ABSTRACT

A novel filler treatment comprising the preparation of swollen starch-latex compositions, prepared in the presence or absence of co-additives, and the addition of the said composition to a filler suspension, has been developed. Use of the treated filler during papermaking improves filler retention and produces filled papers where addition of the filler has only a minimal negative effect on strength properties. The swollen starch-latex compositions can be prepared in a batch or jet cooker, or by mixing with hot water under controlled conditions (i.e., temperature, pH, mixing, mixing time) in order to make the starch granules swell sufficiently to improve their properties as a filler additive but avoiding excess swelling leading to their rupture. The swollen starch-latex composition is then rapidly mixed with the filler slurry, preferably in a static mixer, and added to the papermaking furnish at a point prior to the headbox of the paper machine. The starch-latex composition can be used with wood-free or wood-containing furnishes. The treated filler is easily retained in the web during papermaking, improves drainage, and gives sheets having good formation. Sheets made with the treated fillers have higher bonding and tensile strengths than sheets produced using filler treated with either swollen starch alone or latex alone. Retention and drainage are further improved when conventional retention aid chemicals are added to the furnish containing the treated filler. The use of swollen starch-latex compositions could allow the papermaker to increase the filler content of the paper without sacrificing dry strength properties or increasing the amount, and hence the cost, of the retention aid added. The combination of swollen starch and latex could be used as furnish additives in the manufacture of both filled grades and grades that contain no filler such as sack papers and paperboard products.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Provisional Application Ser. No.60/370,696 Apr. 9, 2002 and the benefit under 35 USC 119(e) of such U.S.Provisional Application is claimed.

BACKGROUND OF THE INVENTION

a) Field of the Invention

This invention relates to filler treatment with swollen starch-latexcompositions, prepared in the presence or absence of co-additives, foruse in the manufacture of filled wood-free and wood-containing papergrades. The invention also relates to combinations of swollen starch andlatex for use as furnish additives in grades that contain no filler suchas sack papers and paperboard products.

b) Description of Prior Art

In the manufacture of filled papers the filler slurry is added to thepulp suspension before it is transferred to the forming section of thepaper machine. A retention aid or retention aid system comprisingseveral components, is always added to the pulp/filler suspension (alsoknown as the furnish) to retain as much of the filler as possible in thesheet. Adding filler to paper provides the papermaker with numerousimprovements in sheet properties, including improved opacity,brightness, feel, and print definition. Furthermore, when the filler ischeaper than the pulp, addition of filler to the sheet results in costsavings due to the replacement of the fibre by filler. These savings canbe substantial when low cost fillers, such as precipitated calciumcarbonate (PCC), are used to replace expensive chemical pulp fibres.Moreover, filled paper is much easier to dry than paper with no fillerand, as a result, the papermachine can run faster with less steamconsumption, which reduces costs and improves productivity. Therefore,the addition of high levels of PCC to the sheet would drastically reducethe cost of fine paper manufacture.

However, for a given sheet weight there are limits to the amount offiller that can be added. The strength of paper is usually the mostimportant factor limiting the filler content, although other factorssuch as retention, drainage, and the chemical demand for retention andsizing are also a consideration.

Making paper with a high filler content requires an efficient retentionaid system. It is required that the retention aid gives good fillerretention under the high shear and turbulence levels found in thepapermachine and that it improve drainage, but without impairingformation. The retention aid chemicals are added to the papermakingfurnish, at a point prior to or at the inlet of the headbox of the papermachine. The retention aids are composed of single or dual chemicaladditives that improve filler and fines retention by a bridging and/orflocculation mechanism. The chemicals help attach the filler particlesand fines (small fibrous fragments) to the long fibres or cause theiraggregation into larger flocculated particles which are more easilyretained in the web. In order to create the attachment and flocculationthe chemicals must adsorb on the surfaces of the fillers, fines andfibres. The degree of adsorption of chemicals and the attachment forcesare influenced by many things including furnish cleanliness and furnishchemistry, the properties of the added chemicals, the level of shear inthe papermaking process and the contact time between the retention aidsand the furnish components.

Paper strength is inevitably reduced by replacement of the fibres byfiller; not only because there are less fibres in the sheet whichreduces the number of fibre-fibre bonds in the sheet, but also becausethe presence of the filler reduces the area of contact between theremaining fibres. Filler particles do not bond between themselves andtheir location at the fibre-fibre bonded area prevents hydrogen bondingfrom occurring between the pulp fibres. As a result, retaining highamounts of filler produces a weaker sheet that can break more easily onthe paper machine, size press, coater, winders and printing presses.Weaker fibre-fibre bonding also decreases the surface strength of thepaper, causing a reduction in pick resistance and an increase inlinting. Poor bonding of filler particles in the fibrous structure canalso increase dusting in the pressroom.

In general, all common inorganic fillers, (for example, clay, groundcalcium carbonate (GCC), PCC, chalk, talc, titanium dioxide,precipitated calcium sulphate, are known to impair strength and increasedemand for chemicals. In particular, fillers with high surface areas,such as scalenohedral PCC which is widely used in the manufacture offine papers, have excessive negative effects on strength and increasethe chemical demand of additives used for strength, sizing andretention. Due to its shape, narrow particle size distribution, and highsurface area, PCC has a tendency to reduce bonding in the sheet morethan other common papermaking fillers, such as chalk, GCC and clay, andalso gives the sheet an open structure which makes the sheet excessivelypermeable or porous. High sheet porosity is detrimental for printquality and liquid absorbency. As the content of PCC is increased in thefurnish the demand for sizing chemicals, such as alkyl ketene dimer(AKD) and alkenyl succinic anhydride (ASA) is increased to maintain thedesired degree of sizing or water repellence. This is because adisproportionate fraction of the sizing chemical is adsorbed on the highsurface area PCC. Poor sizing efficiency and loss of water repellenceover time (size reversion) are common problems associated with the useof PCC in highly-filled wood free papers sized with AKD and ASA. Inrecent years many paper mills making wood-containing grades haveconverted to neutral papermaking to allow use of bright calciumcarbonate fillers, such as GCC and PCC, and major concerns with the useof PCC in these grades remains retention, sheet strength and printingoperations.

An ongoing industry trend is to decrease sheet grammage to reduce costs.Unfortunately, as the grammage is decreased nearly all paper propertiesdeteriorate, including the limiting factors of opacity, bendingstiffness and permeability. Reduction in grammage may also decreaseretention of filler during papermaking and increase the frequency ofsheet breaks both on the paper machine and during converting andprinting. To overcome the loss in sheet opacity the papermaker can addmore of the high opacity fillers, but this in turn can cause furtherdeterioration in sheet strength. The industry needs cost-efficienttechnology for the production of the lightweight grades with good fillerretention and drainage and acceptable strength, formation, optical, andprinting characteristics.

Water-soluble natural and synthetic polymers are commonly used forstrength development in the manufacture of filled and unfilled papergrades. Starch is the oldest and most widely used additive forincreasing the strength of paper. In order to increase strength thestarch macromolecules must adsorb on the long fibres and reinforce thefibre-fibre bonded areas. Cationic and amphoteric starches are added tothe paper machine wet-end in the production of coated and uncoatedwood-free fine papers, bleached paperboards, and many filled- andunfilled-grades. Since starch is inexpensive compared to syntheticpolymers its dosage level can be as high as 40 kg per ton or more.Cationic starch is also used in the preparation of dispersions of AKD,ASA and rosin sizes, and as a retention aid in combination with a silicamicro-particle such as anionic colloidal silicic acid. The cationicstarch or cationic starch-size dispersions are usually easily adsorbedon the negatively-charged fibres and fines and are retained in the sheetduring the forming process. Unfortunately, when cationic starch is usedin chemical pulp furnishes the improvement in the strength of the paperis often low and addition of higher levels of starch does not improvestrength. This phenomenon is related to the limited amount of starchthat can adsorb on the fibres. It appears once the negative charge onthe fibre surfaces is neutralised by the cationic charge of the starchmacromolecules, no further starch adsorbs, even at high dosage rates.With mechanical pulp furnishes the starch performance is usually reducedby the high levels of fibre fines and anionic colloidal solids. Theanionic colloidal solids, also known as anionic trash or dissolved andcolloidal substances (DCS), can neutralise a large portion of thecationic charge on the starch making it ineffective for improvingfibre-fibre bonding. The application of starch in the manufacture offilled wood-free and some wood-containing grades is often limited to amaximum of 4 to 10 kg/ton of paper. At higher dosage rates the starchmay impair drainage and other sheet qualities, such as formation,porosity and brightness, and the improvements in tensile strength areusually small. At present, there are no cost-efficient polymers capableof developing adequate strength when added to furnishes containing highlevels of fines and DCS such as found in mechanical pulp furnishes.

Cationic starch is normally used as a papermaking additive after beingfully cooked. Generally, the starch powder is dispersed in cold water atabout 2%-6% concentration then cooked or gelatinized either in batchcookers at 96° C.-100° C. for a period of about half an hour or injet-cookers at 120-140° C. for a few minutes. These cooking processeslead to complete gelatinization of the starch granules followed by theirdissolution into amylose and amylopectin macromolecules. In specialapplications, such as in the manufacture of heavy paperboard products,the dispersed starch granules are also applied directly to the formedsheet by spraying the uncooked starch slurry onto the moist web. Thegelatinisation of the starch granules is believed to take place duringthe drying operation of the sheet. An improvement in the starch cookingprocess for use in the manufacture of paper was disclosed more thanforty years ago in U.S. Pat. No. 2,805,966, which describes the cookingof the starch slurry in a steam injection cooker. This was said topermit control of the heating so that the majority of the starchgranules were swollen but not ruptured. Two other methods to produce aswollen starch whose granules do not disintegrate during agitation weredisclosed in U.S. Pat. No. 2,113,034 and U.S. Pat. No. 2,328,537. InU.S. Pat. No. 2,113,034 this was accomplished by reaction of the starchwith formaldehyde. In U.S. Pat. No. 2,328,537 this was accomplished byreaction of starch with certain antimony or phosphorous chlorides oroxychlorides. The patents suggest that the products might be useful inmanufacture of paper. However, since the products have limited swellingcharacteristics even in hot water and are only partially retained in thepaper sheet they never found acceptance in the paper manufacturingindustry. U.S. Pat. No. 5,620,510 also discloses a method forpreparation of a swollen starch for use as a dry strength additive inthe papermaking process. In this invention the swollen granules ofstarch were produced under controlled conditions of temperature and pHthat prevent their disintegration during agitation. An alternativemethod of producing swollen modified starch for increasing strength ofpaper was disclosed in patent WO 97/46591. The modified starch isprepared by a process comprising the step of swelling a cationisedcross-linked starch under conditions selected so that a cross-linkingagent, sodium trimetaphosphate, maintains the viscosity of the swollenproduct at a level less than 400 cps. The washed swollen product is tobe added to the paper furnish, at or prior to the headbox of the papermachine. The swollen starches of all the above patents were proposed foraddition to a paper machine pulp furnish.

It has been common knowledge in the paper industry that the addition ofan anionic latex to a papermaking furnish, combined with alum (aluminumchloride), causes the latex to precipitate and thereby give increasedstrength to paperboard. A number of patents, particularly U.S. Pat. No.4,178,205, U.S. Pat. No. 4,189,345, and U.S. Pat. No. 4,187,142,disclose the general idea that a cationic latex can be added to thepapermaking furnish. Because of the anionic nature of the pulp furnish,cationic latex adsorbs easily on pulp surfaces and provide additionalfibre-fibre bonding and tensile strength to the paper product. Thesepatents relate primarily to so-called “high-strength” papers, which arelargely made without addition of fillers. The furnishes of these papergrades contain many other additives including starch, size, alum, andretention aids. Therefore, the strengthening benefits from the additionof latex might be attributed to its interaction with these additives. Inown laboratory handsheet studies on pulp suspensions containing no otheradditives, it has been found that at equal dosage levels the cationiclatices are about 10 times less efficient than a cooked cationic starchin increasing strength. For example, an addition level of 1% cationicstarch, on pulp, produced a greater internal bond strength and tensilestrength than the dosage of 10% cationic latex, despite the large amountof latex being retained in the sheet. Similar low strength results werealso obtained when anionic latices were added to pulp suspensions wherethe fibres have previously been made cationic to promote latexadsorption.

Another approach for improving filler retention, strength and sizingperformance is by treating the filler suspension with additives prior tomixing with the pulp stock. For example, several patents including U.S.Pat. No. 4,225,383, U.S. Pat. No. 4,115,187, U.S. Pat. No. 4,445,970,U.S. Pat. No. 5,514,212, GB Patent 2,016,498, U.S. Pat. No. 4,710,270,and GB Patent 1,505,641 describe the benefits of filler treatment withadditives on retention and sheet properties. It is known that since mostcommon inorganic filler particles in suspension carry a negative charge,the cationic additive adsorbs on their surfaces by electrostaticinteractions causing their agglomeration or flocculation. For anionicadditives to promote flocculation the filler particles would require apositive charge to allow adsorption of the anionic additive. Theflocculation of the filler particles usually improves retention duringsheet making and also increases sheet strength, but excessiveflocculation of filler can also decrease the gain in optical propertiesexpected from the filler addition. GB Patent 2,016,498 disclosesflocculating fillers with a composition comprising cooked starch, anorganic polyelectrolyte, and an agent for controlling the degree offlocculation and viscosity of dispersion. The resulting pre-flocculatedfiller is disclosed to provide improved tensile strength in filledpaper. U.S. Pat. No. 4,710,270 discloses pre-flocculated fillerparticles covered with a dispersion of cationic starch andcarboxymethylcellulose or alginate, resulting in improved strength andretention.

GB Patent 1,505,641 discloses treating calcium carbonate filler withanionic styrene-butadiene latex. Pre-treatment of calcium carbonatefiller, especially chalk whiting, with this latex is used to produceprotected filler particles, which are then added in papermaking toimprove the strength of the filled sheet. This patent also disclosesthat the calcium carbonate filler has a positive zeta-potential,produced by a pre-treatment of the filler with a small amount of a fullycooked cationic starch. The filler particles are made cationic by theaddition of the starch with the objective to promote the adsorption ofthe anionic latex on the surfaces of filler particles. The latex-treatedfiller suspension, containing up to 20 parts of latex per 100 parts ofchalk, is added before the headbox of the paper machine, for example, tothe beater or pulper, and has a smaller negative effect on strengthcompared to untreated filler. Similarly, U.S. Pat. No. 4,445,970discloses a method of manufacturing paper containing a mixture of clayand talc fillers and anionic latex to promote strength. The latex ispreferably added to the machine chest, most preferably in amountsranging between 3 and 7% based on the dry furnish.

At no point do any of the above patents disclose that the starch can beswollen in the presence of latex, either anionic or cationic, with orwithout the use of co-additives, for enhancing bridging between theswollen starch granules and the latex. Also, there are no references orclaims related to the combination of swollen starch and latex in fillertreatment for use in the manufacture of paper or as additives to thefurnish used in papermaking.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a composition for use inpapermaking.

It is another object of the invention to provide a filler suspension foruse in papermaking.

It is yet another object of the invention to provide a pulp furnish forpapermaking.

It is still another object of the invention to provide a process forproviding a filler suspenson for papermaking.

Yet another object of the invention is to provide a method of makingpaper.

Still another object of the invention is to provide a novel paper.

In accordance with one aspect of the invention, there is provided acomposition for use in papermaking comprising swollen starch granulesand a latex in an aqueous vehicle In accordance with another aspect ofthe invention, there is provided a filler suspension for use inpapermaking comprising: filler particles, swollen starch granules and alatex, in an aqueous vehicle.

In accordance with yet another aspect of the invention, there isprovided a pulp furnish for papermaking comprising: pulp fibres, fillerparticles, swollen starch granules and a latex, in an aqueous vehicle.

In accordance with still another aspect of the invention, there isprovided a process for producing a filler suspension for papermakingcomprising contacting particulate filler with swollen starch and alatex, in an aqueous vehicle.

In accordance with yet another aspect of the invention, there isprovided a process of making paper comprising: a) forming an aqueouspapermaking furnish comprising pulp fibers, filler particles, swollenstarch granules and a latex, in an aqueous vehicle, b) draining thefurnish through a screen to form a sheet, and c) drying the sheet.

In accordance with still another aspect of the invention, there isprovided a paper comprising a matrix of pulp fibres and fillerparticles, a retention system for said filler derived from the solidscontent of swollen starch granules and a latex.

DETAILED DESCRIPTION

In one embodiment of the invention there is provided a filler suspensionfor use with pulp fibres in papermaking, comprising filler particles ina liquid vehicle, typically aqueous, and a swollen starch and latex insaid vehicle; the suspension may also contain co-additives for thestarch and latex, and papermaking agents.

In another embodiment of the invention, there is provided a compositionfor treating fillers in papermaking, comprising a swollen starch andlatex; the composition may also contain co-additives for the starch andlatex, and papermaking agents.

In another embodiment of the invention, there is provided a paperfurnish comprising, in an aqueous furnish vehicle, pulp fibres, fillerparticles, a swollen starch and latex; the furnish may also containco-additives for the starch and latex, and papermaking agents.

In still another embodiment of the invention, there is provided a methodof producing paper by adding a filler suspension comprising fillerparticles, a swollen starch and latexin a liquid vehicle, to a pulpfibre stock to form a paper furnish, and producing paper from thefurnish. Anionic and cationic agents can be added to the furnishcontaining the treated filler to enhance retention and improve drainage.The furnish may also contain co-additives for the starch and latex, andpapermaking agents.

The invention also provides processes for producing swollen starch-latexcompositions and their introduction to the filler suspensions.

The swollen starch-latex composition, in the presence or absence ofco-additives, is suitably prepared in batch or jet cookers or by mixingthe suspension of starch and latex with hot water. For a given starch,the swelling is done under controlled conditions of temperature, pH,mixing and mixing time, in order to avoid rupture of the swollen starchgranules. The composition is rapidly added to the filler suspension,which is then introduced to the paper furnish, at a point prior to or atthe headbox of the paper machine. During the drying operation theretained swollen starch granules with filler particles will rupture,thereby liberating amylopectin and amylose macromolecules to bond thesolid components of the sheet.

The combination of swollen starch and latex can be used in fillertreatments and in papermaking under acid, neutral or alkalineenvironments. The compositions are for use in filler treatments suchthat the treated filler is well retained in the sheet and has a minimumnegative influence upon sheet strength. Using the swollen starch-latexcompositions for filler treatment, with or without co-additives, givesgreater retention and strength benefits than the use of the swollenstarch alone or latex alone, or the standard approach of adding fullycooked starch. It was also found that adding swollen starch to thefiller slurry followed by addition of latex produces useful treatedfiller suspensions for use in the manufacture of filled papers. Thecombination of swollen starch and latex was also found useful for use asan additive to filled or unfilled furnishes for strength development.

When the filler is treated with a swollen starch-latex composition, madewith or without co-additives, of this invention and added to a pulpslurry, the filler particles agglomerate and the agglomerated fillerparticles adsorb on the surfaces of the fines and fibres causing theirrapid flocculation in the furnish. This results in good retention of thefiller and fines and improves web drainage even without the addition ofa retention aid. However, under high levels of shear, turbulence andvacuum, filler retention can be reduced due to deflocculation anddetachment of the filler from the fibre surfaces. Adding an anionicmicro-particle, such as colloidal silicic acid, to the papermakingfurnish containing the treated filler, at or prior to the headbox, andpreferably to the pressure screen of paper machine, substantiallyenhanced retention and drainage.

It was surprising to find that the addition of latex to uncooked starch,followed by partial cooking at temperatures slightly below the gel pointto produce swollen starch was a better additive system than addingeither swollen starch alone or latex alone.

It has been found that making the filler particles cationic by treatingthem with water-soluble cationic polymers, including cooked cationicstarch, polyethylenimine, polydadmac or polyvinylamine helped theadsorption of anionic latex on their surfaces. However, the improvementsin strength of filled paper were significantly lower than those achievedin the present invention using the swollen starch-latex compositions.The preparation of swollen starch-latex compositions or cookedstarch-latex complexes, either for use in filler treatment prior tobeing added to pulp suspension in the manufacture of filled papers orfor use as pulp stock additives in the manufacture of paper andpaperboard grades that contain no filler, has not been previouslydisclosed.

a) Fillers

The fillers in accordance with this invention are typically inorganicmaterials having an average particle size ranging from 0.5 to 30 μm,more usually 1 to 10 microns, such as common papermaking fillers likeclay, ground calcium carbonate (GCC), chalk, precipitated calciumcarbonate (PCC), talc, and precipitated calcium sulphate (PCS) and theirblends. The papermaking pulp slurry to which the treated filler is to beadded, in accordance with this invention, can be composed of mechanicalpulp, chemical pulp or recycled pulp and their mixtures.

While the filler generally may comprise particles having sizes in thegeneral range of 0.5 to 30 μm, generally the particles will fall withinthe lower size range of 1 to 10 microns, and the filler particles aregenerally significantly smaller than the swollen starch granules.

b) Swollen Starch Granules

Starches suitable for use in this invention include starch originatingfrom corn, waxy corn, potato, wheat, tapioca, sorghum, waxy sorghum,rice. The starch can be cationic (positively-charged), anionic(negatively-charged), amphoteric (a combination of positive and negativecharges), converted, or unmodified. The average particular size of mostunswollen granules range between 5 and 35 μm.

Starch granules are insoluble in cold water because of their organizedhydrogen-bonded structure. To disperse or “cook” starch, it is necessaryto introduce enough energy to disrupt hydrogen bonding and to introducemolecules of water. When aqueous suspensions of starch are heated, thegranules pass first through a stage of slight, reversible swelling untila critical temperature is reached. At this temperature, known as thepasting or gelatinization temperature, the granular structure “melts”.Massive swelling occurs, which causes a large increase in viscosity.Beyond this stage the viscosity decreases again due to the rupture ofthe swollen granules. Each variety of starch has a different pastingtemperature range. Swollen starch granules used in the invention aredistinct from cooked starch. Cooked starch results when swollen starchgranules disrupt above the gelatinization point of the starch,whereafter the formal amylose and amylopectin dissolve in aqueousmedium.

Depending on the starch source, the particle size of the swollen starchgranules range between 5 μm and 90 μm or higher. The best performance isobtained when the swollen starch granules are carefully controlled toprevent their rupture. The preferred range of the granules is such that80% of the swollen particles are within the range of 30 to 70 μm.

c) Latex

An important aspect of the present invention is the use of a suitablelatex. The latex can be anionic, cationic or amphoteric.

Suitable latices include acrylic latex, cationic styrene/butylacrylatedispersion. Carboxylated styrene/butadiene dispersion, polyvinylacetatedispersion, cationic styrene/butadiene dispersion, n-butylacrylate-acrylonitrile-styrene copolymer. The average particle size oflatexes may range between 800 to 1300 nm.

d) Swollen Starch-latex Composition

The swollen starch granules and the latex interact such that thegranules become carriers for the latex. In particular, anionic chemicalgroups, for example carboxy groups on the latex particles, may interactwith the cationic sites on swollen starch granules to bind latexparticles to the granules. The latex particlesmay be absorbed on theswollen granules.

In general, the compositions comprise 60 to 95%, by weight of thegranules and 40 to 5%, by weight of latex, to a total of 100%, byweight, based on the total solids content of the granules and latex. Thepreparations of starch and latex in the composition depend on the fillerto be treated and the grade of paper to be produced.

It will be understood that the composition will contain complexes oflatex bound to swollen starch granules, as well as free swollen starchgranules and free latex particles.

The composition of swollen starch granules and latex is suitablyemployed in an amount of 1 to 10%, by weight, as dry solids, based onthe weight of filler particles.

e) Co-additives

The compositions of the invention may optionally include co-additivesfor the swollen starch granules and latex, which co-additives enhancethe effectiveness of the composition of starch granules and latex.Typically, the co-additives are anionic, for examplecarboxymethylcellulose, polyacrylic acid, alginate, colloidal silicicacid, bentonite polyacrylamide and soluble soap, or cationic, forexample polyethylene imine, chitosan, polyvinylamine, poly (dadmac),alum, trivalence and tetravalence cations.

In general, where a co-additive is employed, it is present in an amountof 1% to 10%, by weight, suitably 0.5% to 5%, by weight, based on thetotal solids weight of the swollen starch granules and latex.

f) Papermaking Agents

The compositions, suspensions and furnishes of the invention mayadditionally include conventional papermaking agents, for example sizingagents such as alkylketene dimmer, alkenyl succinic anhydride and rosin;wet strength agents, and cationic or anionic polymeric retention aids.The composition may include a retention aid which may be a singlechemical, such as an anionic micro-particle (colloidal silicic acid,bentonite), anionic polyacrylamide, a cationic polymer (cationicpolyacrylamide, cationic starch), or dual chemical systems (cationicpolymer/anionic micro-particle, cationic polymer/anionic polymer). Thechoice of retention aid chemicals and their addition points in the papermachine approach system will depend on the nature of the ionic charge ofthe treated filler slurry and the papermaking furnish.

g) Components of Composition

The choice of preferred starch, latex, and optional co-additives, forthe preparation of the swollen starch-latex compositions, and theiraddition to the filler slurry depends on the nature of the ionic chargeof the starch and latex used as well as the nature of the surface chargeof the filler to be treated. For example, with a cationic starch it ispreferable to use an anionic latex whereas with an anionic or amphotericstarch it is preferable to use a cationic latex. When cationic latex isused with cationic starch following the treatment of the filler, ananionic polymer such as OMC, polyacrylate or alginate or an anionicmicroparticle such as silica or bentonite, can be added to promotebridging between the filler particles and the formation of microflocs.If anionic latex is to be used with an anionic or amphoteric starch thena cationic agent such as those described above, is needed to form acomplex and promote bridging between the treated filler particles.

In order to achieve the best combination of starch, latex and optionalco-additives for a filler treatment it is important to consider thenature of charges of starch and latex as well as the nature of charge ofthe filler to be treated. For a cationic starch or an amphoteric starch,depending of the charge of the latex (anionic or cationic) theco-additive may be cationic or anionic.

In general, the treated filler particles of the invention comprising thefiller particles in the swollen starch granules and the latex areemployed in an amount of 5% to 60%, as dry solids, based on the dryweight of pulp in furnish.

It was found that handsheets made with PCC or clay fillers treated withthe swollen starch-latex compositions, even at a filler content of 40%,had greater internal bond strength, as measured by the Scott bondtechnique, than a control sheet made with no filler. At equal fillercontent the tensile properties and air resistance of sheets made withthe treated filler were all improved and were much greater than thosesheets made with the untreated filler.

The use of the swollen starch and latex combinations of this inventionpermits the production of filled papers, such as coated and uncoatedfine papers, super-calendered papers, and newsprint, with minimalstrength loss, improved air resistance and good optical properties. Thefillers treated according to the invention can thus allow papermakersproducing filled papers to raise the filler content of the sheet withoutsacrificing dry strength properties or increasing the cost of theretention aid. In general, the potential benefits from the use of thetreated filler suspensions of the present invention include improvedretention, drainage, strength, opacity and print quality, and reducedusage of expensive reinforcement chemical pulp fibre and the usage ofretention aid.

Under certain conditions the combination of swollen starch and latex mayalso be used to efficiently strengthen other grades that contain nofiller such as, sack papers and paperboard products.

Particular methods in accordance with the invention include thetreatment of fillers with the additive systems containing swollenstarch, latex. And co-additives. The starch granules in a starch slurryat 2-10% solids and room temperature may be swollen at temperaturesbelow the starch gel point in a batch cooker, a jet cooker or by mixingwith hot water. The preferred method of this invention is to swell thegranules by mixing the starch slurry prepared in cold water with hotwater. The temperature of hot water used depends on the consistency ofthe initial starch slurry in cold water, the temperature of cold waterpH, and residence time to produce the swollen granules. The values oftemperature and reaction time for preparing the swollen starch-latexcomposition would depend on the type of starch used, the pH of thestarch slurry and heating time

-   -   1) A starch dispersion mixed with latex in cold water is        swelled, then the swollen starch-latex composition is added to        an agitated filler suspension. In this method, the starch powder        is first dispersed in cold water then latex is incorporated into        the dispersion under shear. The starch-latex mixture mixed with        hot water or is heated at a temperature below the starch gel        point for a few minutes. The swollen starch-latex composition is        then rapidly mixed with the filler suspension, usually at room        temperature, at consistencies between 10-60%, more preferably        between 20%-40%.    -   2) A starch dispersion is first swelled, then added to an        agitated filler suspension followed by the introduction of        latex. In this method, the starch powder is dispersed in cold        water then mixed with hot water or heated at a temperature below        the starch gel point. The swollen starch is then rapidly mixed        with the filler suspension, usually at room temperature, at        consistencies between 10-60%, more preferably between 20%-40%,        followed by the addition of latex.

The above two polymeric compositions and filler treatment methods wereprepared under good mixing conditions. Anionic agents or cationic agentscan be added during the preparations of swollen starch-latexcompositions to form the complex or to the sheared treated fillersuspension to develop bridged filler particles. By using the rightmixing equipment, these treatment strategies can produce homogeneousfiller suspensions, which are stable during storage for a long period.The viscosity of PCC slurry at a constant concentration, prior to andafter treatment with swollen starch and latex, measured over a widerange of shear, tends to be lower than that of untreated PCC slurry. Thesize of the agregated filler particles can be controlled by the shearforces applied.

The treated filler suspensions can be directly introduced to the pulpslurry or diluted, if needed, and added to the paper machine pulp stockprior to the sheet forming process, i.e., at the blend chest, machinechest, or inlet of the fan pump. The introduction of the treated fillerto the pulp suspension induces flocculation of the pulp slurry. Thedegree of flocculation is, however, influenced by the level of shear andresidence time. In general, the treated-filler suspensions tended toretain their flocculation characteristics over time when added topapermaking pulp slurries. To enhance filler retention an anionicmicro-particle, such as silica, an anionic polymer such as CMC, or aconventional polymeric retention aid such as polyarylamide, can be addedto furnish (comprising the pulp and treated filler), preferably at apoint prior to or at the headbox or pressure screen. Upon addition ofsilica or CMC, to the pulp stock containing the treated filler, theretention and drainage substantially improved.

Microscopy analysis indicates that the filler particles in the form ofsmall aggregates are well distributed in the sheet. The bond strength,tensile properties and air resistance of sheets made with the treatedfiller were all improved and much greater than those of sheets made withthe untreated filler. The treated filler also improved the sheet opticalproperties. It was also found that an increase in the proportion oflatex in the swollen starch-latex compositions used in the treatment offiller dispersions further improved the strength properties of thefilled sheet.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 illustrate schematically processes for preparation of thecompositions containing swollen starch granules and latex, with andwithout co-additives, and for making the treated filler suspension ofthe present invention and its addition to the paper machine pulp stock.

FIGS. 3 and 4 show the internal bond strength (Scott bond) i.e.SBS(J/m²) and breaking length i.e. BL (km) of sheets filled with PCCi.e. PCC % which is the PCC content in the sheets using the conventionalprocess and the process of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1: 1) pulp stock, 2) fan pump, 3) headbox, 4) forming, pressing anddrying sections, 5) filler stock, 6) mixer, pump 8) latex, 9)co-additive, 10) cold water, 11) starch powder, 12) mixing tank, 13)retention aid, 14) paper. Mixing can be done by mechanical agitationwith an impeller or by a centrifugal pump.

In the process of FIG. 1, cold water 10 and starch provider 11 are fedto mixing tank 12, and co-additive 9 is added to mixing tank 12. Thesecomponents are mixed in mixing tank 12 and the resulting mixture is fed,with the addition of latex 8 to mixer 7 for further mixing and swelling.The resulting mixture and filler stock 5 are fed to mixer 6 to produce atreated filler suspension. The filler suspension is added to a pulp frompulp stock 1 and pumped to pump 2 in a pumped flow to headbox 3. Aretention aid 13 is added to the pumped flow. The pulp furnish fromheadbox 3 is fed to the forming, pressing and drying sections 4 of apaper machine from which is received paper 14.

FIG. 2: 1) pulp stock, 2) fan pump, 3) headbox, 4) forming, pressing anddrying sections, 5) filler stock, 6) mixer, 7) mixer, 8) latex, 9)co-additive, 10) cold water, 11) starch powder, 12) mixing tank, 13)retention aid, 14) paper.

The process of FIG. 2 is similar to that of FIG. 1 except that theco-additive 8 is added to the filler suspension from mixer 6 and ismixed with the filler suspension

FIGS. 3 and 4: A) sheets filled with PCC without polymer addition, B)sheets filled with PCC with addition of 1.5% starch to furnish(pulp+PCC)−conventional process, C) sheets filled with PCC treated inaccordance with the invention (swollen starch/latex/co-additive).

EXAMPLES

The method of this invention can be best described and understood by thefollowing illustrative examples. In the examples, the results wereobtained using laboratory scale techniques. The basic procedureconsisted of adding an amount of the treated filler suspension to a pulpslurry under mixing prior to a retention test and sheet making. Thefiller retention test was done using a dynamic retention/drainage jar(also called the Britt Jar) at room temperature. For the Britt Jarretention test, the consistency of the furnish (pulp/filler) was 0.5%,by weight, and the speed of the propeller in the jar was 750 rpm. Thepaper sheets (60 g/m²) were prepared at 50° C. using a dynamic sheetmachine. They were prepared from agitated pulp suspensions containinguntreated or treated PCC. The PCC has an average particle size of 1.6 μmwhich was obtained from Specialty Minerals Inc. Immediately before sheetmaking the furnish was diluted in the sheet machine deckle from 1% to0.1% under shear. The formed moist webs were pressed on a laboratoryroll press to about 40% solids and dried on a photographic dryer. Priorto physical testing, the dried sheets were conditioned in a room at 50%RH and 22° C. for 24 hours.

In the following examples the treated filler suspensions were preparedas follows. A 2% dispersion of cationic waxy maize starch powder (Cato232 trade-mark, from National Starch and Chemical Corporation) wasprepared in cold water then a portion of anionic acrylic copolymer latex(Acronal S866 trade-mark, from BASF) or cationic styrene butylacrylateco-polymer latex (Latex 8675, from BASF) was incorporated into thedispersion under gentle agitation. The homogeneous starch-latex mixturewas then agitated in a heated water bath. When the temperature of thestarch-latex mixture attained 65° C., the mixing was allowed to continuefor few minutes. After this period the swollen starch-latex blend wasrapidly added to the cold filler slurry at 20% solids under mixing withan impeller. Another treated filler suspension was also prepared underthe same conditions, except that the cationic waxy maize starch powderwas added to a 0.1% CMC solution. The CMC used in the examples was atechnical grade obtained from Aldrich. Its average molecular weight was700,000 and degree of substitution was 0.9. The mixture of swollenstarch-CMC-latex was then added to the filler slurry at 20% solids undermixing with an impeller. The fillers of the above two treatments werethen added to 1% pulp suspensions under mixing prior to carrying out theretention test and handsheet making. In some tests, anionicmicro-particle silica (Eka NP890, from Eka Nobel) was added to thesheared furnish during the retention test and handsheet making. Thesilica allowed the production of a micro-flocculated furnish andimproved retention and formation. Some filler samples were treated withswollen starch alone or latex alone and used to make filled sheets forcomparison.

Three pulps were used in the following examples. One was aperoxide-bleached thermo-mechanical pulp (BTMP). Another was collectedfrom a mixed stock of a paper machine producing fine paper and wascomposed of bleached hardwood kraft pulp (BHKP), bleached softwood kraftpulp (BSKP) and recycled pulp. This pulp had a Canadian StandardFreeness (CSF) of 540 mL CSF. The third pulp stock was prepared in thelaboratory from 80% BHKP and 20% BSKP and had a freeness of 560 mL CSF.The pH was 5.5 for all furnishes containing clay filler and 7.0 and 8.3for BTMP and fine paper furnishes, respectively, blended withprecipitated calcium carbonate filler.

Example 1

PCC retention results were obtained in the Britt jar at 750 rpm, usingPCC treated with different polymeric combinations. Furnishes at 0.5%solids were prepared by mixing PCC treated with swollen cationic starch(Cato 232), made in the absence and presence of CMC and latex, with BTMPsuspensions at 50° C. and under shear. The amount of treated PCC in thefurnish was 40%. At the same dosage levels the retention of PCC was low(i.e., less than 30%) when the PCC was treated with swollen starch,anionic latex (Acronal S866) or cationic latex (Latex 8675). The PCCretention ranged between 75 and 80% when the PCC was treated withswollen starch-latex compositions made with or without co-additive CMC.It was over 90% when the filler treated with swollen starch-latexcompositions, prepared in the presence or absence of CMC, was added topulp suspension then followed by addition of anionic silica (0.1 to 0.2%based on furnish). Similar retention results were obtained with kaolinclay. Similar high PCC retention values were also measured when the PCCtreated with swollen starch-latex, in absence or presence of CMC, wasadded to fine paper furnishes followed by addition of silica. A Veryhigh retention was also achieved when the silica was replaced by CMC.

Example 2

Table 1 presents the properties of 60 g/m² BTMP sheets made with andwithout PCC (treated and untreated) at pH 7.0. The sheet were allcalendered under the same conditions (80 kN/m and 50 ° C.). Thetreatment of PCC had no detrimental effect on sheet opacity. The contentof PCC in the 60 g/m² sheets varied between 23 and 39%. The sheets madewith the addition of untreated PCC (27% in sheet) have higher airpermeability (more open sheet) and have lower internal bond strength(Scott bond) and burst strength. The treatment of PCC with swollencationic starch-latex compositions, in the presence or absence of CMC,reduced the air permeability of the sheet and substantially improved theScott bond strength and burst strength. The treatment of PCC with theswollen cationic starch-anionic latex compositions increased the bondstrength of the filled sheet even at high filler content. A lesspermeable sheet is more desirable because of reduced absorbency ofliquids in ink, coating and sizing applications. However, a commonproblem associated with the aggregation of filler particles is areduction in the light scattering power leading to reduced sheetopacity. The opacity of sheets made with untreated PCC or treated PCCare quite similar, suggesting that the treatment did not significantlyimpaired the light scattering power of PCC. Similar trend results wereobtained when PCC was replaced by kaolin clay fillers.

TABLE 1 BTMP sheet. Calendered at 50° C. and 80 kN/m. Internal bond PCCin strength, Burst index Gurley AR Addition conditions sheet, % J/m² kPa· m²/g (s/100 mL) Opacity, % No PCC 0 150 1.76 114 86 Non treated PCC 27100 0.99 82 92 39 78 0.75 58 92 PCC treated with 34 215 1.18 107 92 10%CS/2.5% AL PCC treated with 33 238 1.38 129 92 10% CS/5% AL PCC treatedwith 36 242 1.34 165 91 10% CS/0.1% CMC/2.5% AL PCC: precipitatedcalcium carbonate, CS: swollen cationic starch, AL: anionic latex.Except when indicated otherwise, amounts in % are to be understood as %by weight.

Example 3

Table 2 presents the properties of wood-free sheets made with andwithout addition of 25% PCC (treated and untreated) to a pulp furnish #1(collected from a mixed stock of a paper machine producing fine paper)at pH 8.3. The treatment of PCC with the swollen cationic starchfollowed by anionic latex substantially improved the bonding strengthand tensile properties. The sheets having 20% PCC have greater bondingstrength then the sheets made without PCC (control sample). Thetreatment of PCC with the blend has small negative effect on the gain inbrightness and opacity due to a small reduction in the light scatteringcoefficient of the filler.

TABLE 2 Fine paper sheets - mill pulp furnish #1 No Non 5% CS 5% CS/1%AL 5% CS/5% AL Properties PCC treatment on PCC on PCC on PCC PCC content0 20.9 19.1 20.0 24.3 in sheet, % Internal bond 225 91 145 203 243strength, J/m² Breaking 4.71 2.08 2.64 3.17 3.37 length, km Stretch, %3.37 1.42 2.09 2.43 2.76 Opacity, % 74.5 85.0 85.5 84.8 81.4 Brightness,% 89.3 92.0 90.1 89.9 89.7 PCC: precipitated calcium carbonate, CS:swollen cationic starch, AL: anionic latex,

Example 4

Table 3 presents the properties of wood-free sheets made with andwithout addition of 25% treated PCC to a pulp furnish #2 (collected froma mixed stock of a paper machine producing fine paper) at pH 8.3. ThePCC was treated with swollen cationic starch (CS) alone and threemixtures of CS/AL at 90%/10%, 75%/25%, and 50%/50%. The total dosagelevel of CS, or CS/AL mixtures was 10% on PCC based on dry weight. Thetreatment of PCC with the swollen cationic starch alone improved thebonding strength and tensile properties. However, the treatment of PCCwith a combination of CS and AL substantially improved these properties,the strength properties were much higher than those sheets made with CSalone. The best strength improvement was achieved with PCC treated witha mixture composed of 75% CS and 25% AL. The 60 g/m² sheets made with20% PCC treated with the three CS/AL compositions their ISO opacity is85%, and have greater bonding strength and tensile properties than thesheet made without PCC (control sample with ISO opacity 72%). The sheetsmade with treated PCC have higher tear resistance than sheets made withuntreated PCC. The treatment of PCC with swollen CS alone made the sheetmore permeable, whereas the sheets made with PCC treated using swollenCS/AL tend to become less permeable as the amount of latex in themixture increases.

TABLE 3 Fine paper sheets - mill pulp furnish #2 Internal bond TearAdditions PCC in strength, B.L., index, Brightness, Opacity, conditionssheet, % J/m² km Stretch, % mNm2/g % % No PCC 0 309 5.65 3.52 6.64 90.872.3 PCC treated 19.9 212 2.69 1.69 5.52 92.1 85.8 with CS only PCCtreated 17.3 355 4.00 3.15 5.51 91.7 84.3 with CS/AL (90/10) PCC treated20.8 413 4.59 3.52 5.61 91.3 84.4 with CS/AL (75/25) PCC treated 20.8364 4.05 3.59 5.6 91.1 85.5 with CS/AL (50/50) PCC: precipitated calciumcarbonate, CS: cationic starch, AL: anionic latex. B.L.: breakinglength.

Example 5

Table 4 presents the properties of wood-free sheets made with andwithout addition of 20 to 40% PCC to a pulp furnish (made in thelaboratory) at pH 8.3. The PCC was either untreated or treated with amixture of swollen CS (75%) and AL (25%), in the absence and presence of0.125% CMC. The total dosage level of the CS/AL/CMC mixture was 10% onPCC based on dry weight. FIGS. 3 and 4 show that the introduction of PCCto fine paper sheets without addition of strength polymers substantiallyreduced the internal bond strength (Scott bond) and the breaking length.The addition of 1.5% (15 kg/ton) of cooked starch to furnish containingPCC (conventional process) only slightly improved these properties. Thetreatment of PCC with the swollen CS/AL/CMC improved these strengthproperties better than the treatment of PCC with CS/AL. For sheetscontaining 19.5% untreated PCC their Scott bond strength was reduced by52% compared to unfilled sheets and the breaking length dropped by 60%.The sheets made with 23.7% PCC treated with swollen CS/AL mixture hadScott bond strengths that were 36% greater and their breaking lengthdropped by only 31%. The treatment of PCC with combination of CS/AL/CMCsubstantially improved these properties, even at high filler content(FIGS. 3 and 4). For example, the sheets made with 24.3% PCC treatedwith swollen CS/AL/CMC mixture had Scott bond strengths that were 58%greater and their breaking length dropped by only 13%. The sheets madewith 35.6% PCC treated with swollen CS/AL/CMC mixture had Scott bondstrength that were 49% greater and their breaking length dropped by only29%. Therefore, using our invention it was possible to produce sheetscontaining high filler contents and with a Scott bond value greater thanfor the unfilled sheet and with only a small loss in tensile properties.Additionally the optical properties of the sheet were not impaired bythis treatment and the permeability of the sheet was improved.

TABLE 4 Fine paper sheets - lab pulp furnish PCC Internal in bond TearAddition sheet, strength, B.L., Stretch, index, Opacity, conditions %J/m² km % mNm2/g Brightness, % % No PCC 0 107 3.65 1.53 8.77 88.7 73.94Non treated 13.0 69 2.62 1.10 5.05 90.4 84.8 PCC 19.5 51 1.47 0.76 3.8691.2 84.9  1.5% 17.4 80 2.06 1.18 6.05 89.8 84.9 cooked 31.6 59 1.190.72 3.68 91.1 86.2 starch added to furnish (fibre + PCC) PCC treated20.0 133 2.9 1.72 6.40 90.5 82.9 with CS only PCC treated 24.3 169 3.161.78 6.23 89.4 84.2 with swollen 35.6 159 2.49 1.68 5.32 89.9 84.5 CS/AL(75/25) in presence of 0.125% CMC PCC: precipitated calcium carbonate,CS: swollen cationic starch, AL: anionic latex. B.L.: breaking length.

1. A pulp furnish for papermaking comprising: pulp fibres, fillerparticles, and a composition of a complex of ionic, unruptured, swollenstarch granules and an ionic latex, in an aqueous vehicle.
 2. A pulpfurnish according to claim 1, comprising said filler particles, swollenstarch granules and latex in an amount of 1% to 60%, by weight, solids,based on the dry weight of pulp fibres.
 3. A pulp furnish according toclaim 2, comprising wherein said filler particles are inorganicpapermaking filler particles having a size of 1 to 10 microns, and saidswollen starch granules have a size of 5 to 90 microns; said starchgranules and latex being present in an amount of 1 to 10%, by weightsolids, based on the weight of filler, and comprising 60 to 95%, byweight of said granules and 40 to 5%, by weight of latex, to a total of100%, by weight, based on the total solids content of said granules andlatex.
 4. A pulp furnish according to claim 2, wherein said latex haschemical groups, absorbed on said granules; arid said granules are of acationic starch and said latex, is anionic or cationic.
 5. A pulpfurnish according to claim 2, wherein said latex has chemical groups,absorbed on said granules; and said granules are of a anionic oramphoteric starch and said latex is cationic or anionic.
 6. A pulpfurnish according to claim 3, wherein said latex has chemical groupswhich interact with said granules.
 7. A pulp furnish according to claim6, wherein said chemical groups are carboxy groups, cationic groups oramphoteric groups.
 8. A pulp furnish according to claim 1, wherein saidswollen starch granules have a size of 5 to 90 μm.
 9. A pulp furnishaccording to claim 1, wherein said granules are of an cationic starchand said latex is anionic.
 10. A pulp furnish according to claim 1,wherein said starch granules are of an anionic amphoteric starch andlatex is cationic or anionic.
 11. A pulp furnish according to claim 1,comprising 60 to 95%, by weight of said granules and 40 to 5%, by weightof latex, to a total of 100%, by weight, based on the total solidscontent of said granules and latex.
 12. A pulp furnish according to clam11, further including 0.5 to 5%, by weight, of a co-additive, based onthe total solids content of said starch granules and latex, saidco-additive being selected from the group consisting of cationic agentsand anionic agents.
 13. A pulp furnish according to claim 12, whereinsaid co-additive is an anionic agent selected from the group consistingof carboxymethyl cellulose, polyacrylic acid, alginate, colloidalsilicic acid, bentonite and polyacrylamide.
 14. A pulp furnish accordingto claim 12, wherein said co-additive is a cationic agent selected fromthe group consisting of chitosan, polyvinylamine, polyethyleneimine andpoly(dadmac), and alum.
 15. A pulp furnish according to claim 1, furtherincluding a papermaking additive selected from the group consisting ofsizing agents and wet strength agents.